Low resistance thin film organic solar cell electrodes

ABSTRACT

A method which lower the series resistance of photosensitive devices includes providing a transparent film of a first electrically conductive material arranged on a transparent substrate; depositing and patterning a mask over the first electrically conductive material, such that openings in the mask have sloping sides which narrow approaching the substrate; depositing a second electrically conductive material directly onto the first electrically conductive material exposed in the openings of the mask, at least partially filling the openings; stripping the mask, leaving behind reentrant structures of the second electrically conductive material which were formed by the deposits in the openings of the mask; after stripping the mask, depositing a first organic material onto the first electrically conductive material in between the reentrant structures; and directionally depositing a third electrically conductive material over the first organic material deposited in between the reentrant structures, edges of the reentrant structures aligning deposition so that the third electrically conductive material does not directly contact the first electrically conductive material, and does not directly contact the second electrically conductive material.

UNITED STATES GOVERNMENT RIGHTS

This invention was made with U.S. Government support under Contract No.339-6002 awarded by the U.S. Air Force Office of Scientific Research andunder Contract No. 341-4141 awarded by U.S. Department of Energy,National Renewable Energy Laboratory. The government has certain rightsin this invention.

JOINT RESEARCH AGREEMENT

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a jointuniversity-corporation research agreement: Princeton University, TheUniversity of Southern California, and Global Photonic EnergyCorporation. The agreement was in effect on and before the date theclaimed invention was made, and the claimed invention was made as aresult of activities undertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention generally relates to the manufacturing of organicphotosensitive optoelectronic devices. More specifically, it is directedto a method for building an organic photosensitive optoelectronicdevices having a low resistance transparent electrode.

BACKGROUND

Optoelectronic devices rely on the optical and electronic properties ofmaterials to either produce or detect electromagnetic radiationelectronically or to generate electricity from ambient electromagneticradiation.

Photosensitive optoelectronic devices convert electromagnetic radiationinto an electrical signal or electricity. Solar cells, also calledphotovoltaic (“PV”) devices, are a type of photosensitive optoelectronicdevice that is specifically used to generate electrical power.Photoconductor cells are a type of photosensitive optoelectronic devicethat are used in conjunction with signal detection circuitry whichmonitors the resistance of the device to detect changes due to absorbedlight. Photodetectors, which may receive an applied bias voltage, are atype of photosensitive optoelectronic device that are used inconjunction with current detecting circuits which measures the currentgenerated when the photodetector is exposed to electromagneticradiation.

These three classes of photosensitive optoelectronic devices may bedistinguished according to whether a rectifying junction as definedbelow is present and also according to whether the device is operatedwith an external applied voltage, also known as a bias or bias voltage.A photoconductor cell does not have a rectifying junction and isnormally operated with a bias. A PV device has at least one rectifyingjunction and is operated with no bias. A photodetector has at least onerectifying junction and is usually but not always operated with a bias.

As used herein, the term “rectifying” denotes, inter alia, that aninterface has an asymmetric conduction characteristic, i.e., theinterface supports electronic charge transport preferably in onedirection. The term “semiconductor” denotes materials which can conductelectricity when charge carriers are induced by thermal orelectromagnetic excitation. The term “photoconductive” generally relatesto the process in which electromagnetic radiant energy is absorbed andthereby converted to excitation energy of electric charge carriers sothat the carriers can conduct (i.e., transport) electric charge in amaterial. The term “photoconductive material” refers to semiconductormaterials which are utilized for their property of absorbingelectromagnetic radiation to generate electric charge carriers. As usedherein, “top” means furthest away from the substrate, while “bottom”means closest to the substrate. There may be intervening layers, unlessit is specified that the first layer is “in physical contact with” thesecond layer.

When electromagnetic radiation of an appropriate energy is incident uponan organic semiconductor material, a photon can be absorbed to producean excited molecular state. In organic photoconductive materials, thegenerated molecular state is generally believed to be an “exciton,”i.e., an electron-hole pair in a bound state which is transported as aquasi-particle. An exciton can have an appreciable life-time beforegeminate recombination (“quenching”), which refers to the originalelectron and hole recombining with each other (as opposed torecombination with holes or electrons from other pairs). To produce aphotocurrent, the electron-hole forming the exciton are typicallyseparated at a rectifying junction.

In the case of photosensitive devices, the rectifying junction isreferred to as a photovoltaic heterojunction. Types of organicphotovoltaic heterojunctions include a donor-acceptor heterojunctionformed at an interface of a donor material and an acceptor material, anda Schottky-barrier heterojunction formed at the interface of aphotoconductive material and a metal.

FIG. 1 is an energy-level diagram illustrating an example donor-acceptorheterojunction. In the context of organic materials, the terms “donor”and “acceptor” refer to the relative positions of the Highest OccupiedMolecular Orbital (“HOMO”) and Lowest Unoccupied Molecular Orbital(“LUMO”) energy levels of two contacting but different organicmaterials. If the LUMO energy level of one material in contact withanother is lower, then that material is an acceptor. Otherwise it is adonor. It is energetically favorable, in the absence of an externalbias, for electrons at a donor-acceptor junction to move into theacceptor material.

As used herein, a first HOMO or LUMO energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level 10. A higher HOMO energylevel corresponds to an ionization potential (“IP”) having a smallerabsolute energy relative to a vacuum level. Similarly, a higher LUMOenergy level corresponds to an electron affinity (“EA”) having a smallerabsolute energy relative to vacuum level. On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material.

After absorption of a photon 6 in the donor 152 or the acceptor 154creates an exciton 8, the exciton 8 disassociates at the rectifyinginterface. The donor 152 transports the hole (open circle) and theacceptor 154 transports the electron (dark circle).

A significant property in organic semiconductors is carrier mobility.Mobility measures the ease with which a charge carrier can move througha conducting material in response to an electric field. In the contextof organic photosensitive devices, a material that conductspreferentially by electrons due to a high electron mobility may bereferred to as an electron transport material. A material that conductspreferentially by holes due to a high hole mobility may be referred toas a hole transport material. A layer that conducts preferentially byelectrons, due to mobility and/or position in the device, may bereferred to as an electron transport layer (“ETL”). A layer thatconducts preferentially by holes, due to mobility and/or position in thedevice, may be referred to as a hole transport layer (“HTL”).Preferably, but not necessarily, an acceptor material is an electrontransport material and a donor material is a hole transport material.

How to pair two organic photoconductive materials to serve as a donorand an acceptor in a photovoltaic heterojunction based upon carriermobilities and relative HOMO and LUMO levels is well known in the art,and is not addressed here.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic optoelectronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule.” In general, asmall molecule has a defined chemical formula with a molecular weightthat is the same from molecule to molecule, whereas a polymer has adefined chemical formula with a molecular weight that may vary frommolecule to molecule. As used herein, “organic” includes metal complexesof hydrocarbyl and heteroatom-substituted hydrocarbyl ligands.

For additional background explanation and description of the state ofthe art for organic photosensitive devices, including their generalconstruction, characteristics, materials, and features, U.S. Pat. No.6,657,378 to Forrest et al., U.S. Pat. No. 6,580,027 to Forrest et al.,and U.S. Pat. No. 6,352,777 to Bulovic et al. are incorporated herein byreference.

SUMMARY OF THE INVENTION

A method and structure are disclosed which lower the series resistanceof photosensitive devices.

The method includes providing an electrode comprising a firstelectrically conductive material, arranged on a transparent substrate;depositing and patterning a mask over the first electrically conductivematerial, such that openings in the mask have sloping sides which narrowapproaching the substrate; depositing a second electrically conductivematerial directly onto the first electrically conductive materialexposed in the openings of the mask, at least partially filling theopenings; stripping the mask, leaving behind reentrant structures of thesecond electrically conductive material which were formed by thedeposits in the openings of the mask; after stripping the mask,depositing a first organic material onto the first electricallyconductive material in between the reentrant structures; and depositinga third electrically conductive material over the first organic materialdeposited in between the reentrant structures, edges of the reentrantstructures aligning deposition so that the third electrically conductivematerial does not directly contact the first electrically conductivematerial, and does not directly contact the second electricallyconductive material.

The second electrically conductive material may have a similarconductivity to the first electrically conductive material, or may bemore conductive.

The first electrically conductive material of the transparent electrodemay be a metal or a metal substitute. Such metal substitutes include,among other things, oxides and polymers. An example conductive oxideconductive oxide is a degenerate semiconductor oxide.

The second electrically conductive material may be a metal or metalsubstitute.

The method may further include electrically interconnecting the thirdelectrically conductive material deposited between the reentrantstructures. This may be accomplished, among other ways, by forminginterconnects/vias as part of a wiring step, or by structuring thereentrant structures so that the third electrically conductive materialforms a single continuous layer when deposited.

The steps described above may be used to create a donor-acceptorheterojunction or a Schottky-barrier heterojunction. To form adonor-acceptor heterojunction, the method may further include depositinga second organic material over the first organic material, the first andsecond organic materials forming a planar, bulk, or hybriddonor-acceptor heterojunction. Alternatively, the method may furtherinclude depositing a second organic material with the first organicmaterial, the first and second organic materials forming a mixed, bulkor hybrid donor-acceptor heterojunction.

To form a Schottky-barrier heterojunction, the first electricallyconductive material may be metal, the transparent electrode and thefirst organic material forming a Schottky-barrier heterojunction.Alternatively, the third electrically conductive material may be metal,the third electrically conductive material and the first organicmaterial forming a Schottky-barrier heterojunction. An example of astructure that may be used for the transparent electrode is a sheet inphysical contact with the transparent substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an energy level diagram illustrating a donor-acceptorheterojunction.

FIG. 2 illustrates an organic photosensitive device including adonor-acceptor heterojunction.

FIG. 3 illustrates a donor-acceptor bilayer forming a planarheterojunction.

FIG. 4 illustrates a hybrid heterojunction including a mixedheterojunction between a donor layer and an acceptor layer.

FIG. 5 illustrates a bulk heterojunction.

FIG. 6 illustrates an organic photosensitive device including aSchottky-barrier heterojunction.

FIG. 7 illustrates tandem photosensitive cells in series.

FIG. 8 illustrates tandem photosensitive cells in parallel.

FIGS. 9A-9F illustrate a method for forming an organic photovoltaicdevice having low resistance contacts.

FIGS. 10A and 10B illustrate a mask that can be used for the stepillustrated in FIG. 9B that simplifies creation of a common anode andcathode. FIG. 10A is a cross section of the structure, and FIG. 10B isan overhead view, with line A-A′ providing a frame-of-reference betweenthe figures.

FIGS. 11A and 11B illustrate the conductor patterns resulting from theusage of the mask illustrated in FIGS. 10A and 10B. FIG. 11A is a crosssection of the structure, and FIG. 11B is an overhead view, with lineB-B′ providing a frame-of-reference between the figures.

The figures are not necessarily drawn to scale.

DETAILED DESCRIPTION

An organic photosensitive device comprises at least one photoactiveregion in which light is absorbed to form an exciton, which maysubsequently dissociate into an electron and a hole. FIG. 2 shows anexample of an organic photosensitive optoelectronic device 100 in whichthe photoactive region 150 comprises a donor-acceptor heterojunction.The “photoactive region” is a portion of a photosensitive device thatabsorbs electromagnetic radiation to generate excitons that maydissociate in order to generate an electrical current. Device 100comprises an anode 120, an anode smoothing layer 122, a donor 152, anacceptor 154, an exciton blocking layer (“EBL”) 156, and a cathode 170,over a substrate 110.

Examples of EBL 156 are described in U.S. Pat. No. 6,451,415 to Forrestet al., which is incorporated herein by reference for its disclosurerelated to EBLs. Additional background explanation of EBLs may also befound in Peumans et al., “Efficient photon harvesting at high opticalintensities in ultrathin organic double-heterostructure photovoltaicdiodes,” Applied Physics Letters 76, 2650-52 (2000). EBLs reducequenching by preventing excitons from migrating out of the donor and/oracceptor materials.

The terms “electrode” and “contact” are used interchangeably herein torefer to a layer that provides a medium for delivering photo-generatedcurrent to an external circuit or providing a bias current or voltage tothe device. As illustrated in FIG. 2, anode 120 and cathode 170 areexamples. Electrodes may be composed of metals or “metal substitutes.”Herein the term “metal” is used to embrace both materials composed of anelementally pure metal, and also metal alloys which are materialscomposed of two or more elementally pure metals. The term “metalsubstitute” refers to a material that is not a metal within the normaldefinition, but which has the metal-like properties such asconductivity, such as doped wide-bandgap semiconductors, degeneratesemiconductors, conducting oxides, and conductive polymers. Electrodesmay comprise a single layer or multiple layers (a “compound” electrode),may be transparent, semi-transparent, or opaque. Examples of electrodesand electrode materials include those disclosed in U.S. Pat. No.6,352,777 to Bulovic et al., and U.S. Pat. No. 6,420,031, toParthasarathy, et al., each incorporated herein by reference fordisclosure of these respective features. As used herein, a material issaid to be “transparent” if it transmits at least 50% of the ambientelectromagnetic radiation in a relevant wavelength.

The substrate 110 may be any suitable substrate that provides desiredstructural properties. The substrate may be flexible or rigid, planar ornon-planar. The substrate may be transparent, translucent or opaque.Rigid plastics and glass are examples of preferred rigid substratematerials. Flexible plastics and metal foils are examples of preferredflexible substrate materials.

An anode-smoothing layer 122 may be situated between the anode layer 120and the donor layer 152. Anode-smoothing layers are described in U.S.Pat. No. 6,657,378 to Forrest et al., incorporated herein by referencefor its disclosure related to this feature.

In FIG. 2, the photoactive region 150 comprises the donor material 152and the acceptor material 154. Organic materials for use in thephotoactive region may include organometallic compounds, includingcyclometallated organometallic compounds. The term “organometallic” asused herein is as generally understood by one of ordinary skill in theart and as given, for example, in Chapter 13 of “Inorganic Chemistry”(2nd Edition) by Gary L. Miessler and Donald A. Tarr, Prentice Hall(1999).

Organic layers may be fabricated using vacuum deposition, spin coating,organic vapor-phase deposition, inkjet printing and other methods knownin the art.

Examples of various types of donor-acceptor heterojunctions are shown inFIGS. 3-5. FIG. 3 illustrates a donor-acceptor bilayer forming a planarheterojunction. FIG. 4 illustrates a hybrid heterojunction including amixed heterojunction 153 comprising a mixture of donor and acceptormaterials. FIG. 5 illustrates an idealized “bulk” heterojunction. A bulkheterojunction, in the ideal photocurrent case, has a single continuousinterface between the donor material 252 and the acceptor material 254,although multiple interfaces typically exist in actual devices. Mixedand bulk heterojunctions can have multiple donor-acceptor interfaces asa result of having plural domains of material. Domains that aresurrounded by the opposite-type material (e.g. a domain of donormaterial surrounded by acceptor material) may be electrically isolated,such that these domains do not contribute to photocurrent. Other domainsmay be connected by percolation pathways (continuous photocurrentpathways), such that these other domains may contribute to photocurrent.The distinction between a mixed and a bulk heterojunction lies indegrees of phase separation between donor and acceptor materials. In amixed heterojunction, there is very little or no phase separation (thedomains are very small, e.g., less than a few nanometers), whereas in abulk heterojunction, there is significant phase separation (e.g.,forming domains with sizes of a few nanometers to 100 nm).

Small-molecule mixed heterojunctions may be formed, for example, byco-deposition of the donor and acceptor materials using vacuumdeposition or vapor deposition. Small-molecule bulk heterojunctions maybe formed, for example, by controlled growth, co-deposition withpost-deposition annealing, or solution processing Polymer mixed or bulkheterojunctions may be formed, for example, by solution processing ofpolymer blends of donor and acceptor materials.

If a photoactive region includes a mixed layer (153) or bulk layers(252, 254) and one or both of the donor (152) and acceptor layers (154),the photoactive region is said to include a “hybrid” heterojunction. Thearrangement of layers in FIG. 4 is an example. For additionalexplanation of hybrid heterojunctions, U.S. application Ser. No.10/910,371 entitled “High efficiency organic photovoltaic cellsemploying hybridized mixed-planar heterojunctions” by Jiangeng Xue etal., filed Aug. 4, 2004, is hereby incorporated by reference.

In general, planar heterojunctions have good carrier conduction, butpoor exciton dissociation; a mixed layer has poor carrier conduction andgood exciton dissociation, and a bulk heterojunction has good carrierconduction and good exciton dissociation, but may experience chargebuild-up at the end of the material “cul-de-sacs,” lowering efficiency.Unless otherwise stated, planar, mixed, bulk, and hybrid heterojunctionsmay be used interchangeably as donor-acceptor heterojunctions throughoutthe embodiments disclosed herein.

FIG. 6 shows an example of a organic photosensitive optoelectronicdevice 300 in which the photoactive region 350 is part of aSchottky-barrier heterojunction. Device 300 comprises a transparentcontact 320, a photoactive region 350 comprising an organicphotoconductive material 358, and a Schottky contact 370. The Schottkycontact 370 is typically formed as a metal layer. If the photoconductivelayer 358 is an ETL, a high work function metal such as gold may beused, whereas if the photoconductive layer is an HTL, a low workfunction metal such as aluminum, magnesium or indium may be used. In aSchottky-barrier cell, a built-in electric field associated with theSchottky barrier pulls the electron and hole in an exciton apart.Generally, this field-assisted exciton dissociation is not as efficientas the disassociation at a donor-acceptor interface.

The devices as illustrated are connected to an element 190. If thedevice is a photovoltaic device, element 190 is a resistive load whichconsumes or stores power. If the device is a photodetector, element 190is a current detecting circuit which measures the current generated whenthe photodetector is exposed to light, and which may apply a bias to thedevice (as described for example in Published U.S. Patent Application2005-0110007 A1, published May 26, 2005 to Forrest et al.). If therectifying junction is eliminated from the device (e.g., using a singlephotoconductive material as the photoactive region), the resultingstructures may be used as a photoconductor cell, in which case theelement 190 is a signal detection circuit to monitor changes inresistance across the device due to the absorption of light. Unlessotherwise stated, each of these arrangements and modifications may beused for the devices in each of the drawings and embodiments disclosedherein.

An organic photosensitive optoelectronic device may also comprisetransparent charge transfer layers, electrodes, or charge recombinationzones. A charge transfer layer may be organic or inorganic, and may ormay not be photoconductively active. A charge transfer layer is similarto an electrode, but does not have an electrical connection external tothe device and only delivers charge carriers from one subsection of anoptoelectronic device to the adjacent subsection. A charge recombinationzone is similar to a charge transfer layer, but allows for therecombination of electrons and holes between adjacent subsections of anoptoelectronic device. A charge recombination zone may includesemi-transparent metal or metal substitute recombination centerscomprising nanoclusters, nanoparticles, and/or nanorods, as describedfor example in U.S. Pat. No. 6,657,378 to Forrest et al.; U.S. patentapplication Ser. No. 10/915,410 entitled “Organic PhotosensitiveDevices” by Rand et al., filed Aug. 11, 2004; and U.S. patentapplication Ser. No. 10/979,145 entitled “Stacked Organic PhotosensitiveDevices” by Forrest et al., filed Nov. 3, 2004; each incorporated hereinby reference for its disclosure of recombination zone materials andstructures. A charge recombination zone may or may not include atransparent matrix layer in which the recombination centers areembedded. A charge transfer layer, electrode, or charge recombinationzone may serve as a cathode and/or an anode of subsections of theoptoelectronic device. An electrode or charge transfer layer may serveas a Schottky contact.

FIGS. 7 and 8 illustrate examples of tandem devices including suchtransparent charge transfer layers, electrodes, and charge recombinationzones. In device 400 in FIG. 7, photoactive regions 150 and 150′ arestacked electrically in series with an intervening conductive region460. As illustrated without external electrical connections, interveningconductive region 460 may be a charge recombination zone or may be acharge transfer layer. As a recombination zone, region 460 comprisesrecombination centers 461 with or without a transparent matrix layer. Ifthere is no matrix layer, the arrangement of material forming the zonemay not be continuous across the region 460. Device 500 in FIG. 8illustrates photoactive regions 150 and 150′ stacked electrically inparallel, with the top cell being in an inverted configuration (i.e.,cathode-down). In each of FIGS. 7 and 8, the photoactive regions 150 and150′ and blocking layers 156 and 156′ may be formed out of the samerespective materials, or different materials, depending upon theapplication. Likewise, photoactive regions 150 and 150′ may be a sametype (i.e., planar, mixed, bulk, hybrid) of heterojunction, or may be ofdifferent types.

In each of the devices described above, layers may be omitted, such asthe exciton blocking layers. Other layers may be added, such asreflective layers or additional photoactive regions. The order of layersmay be altered or inverted. A concentrator or trapping configuration maybe employed to increase efficiency, as disclosed, for example in U.S.Pat. No. 6,333,458 to Forrest et al. and U.S. Pat. No. 6,440,769 toPeumans et al., which are incorporated herein by reference. Coatings maybe used to focus optical energy into desired regions of a device, asdisclosed, for example in U.S. patent application Ser. No. 10/857,747entitled “Aperiodic dielectric multilayer stack” by Peumans et al.,filed Jun. 1, 2004, which is incorporated herein by reference. In thetandem devices, transparent insulative layers may be formed betweencells, with the electrical connection between the cells being providedvia electrodes. Also in the tandem devices, one or more of thephotoactive regions may be a Schottky-barrier heterojunction instead ofa donor-acceptor heterojunction. Arrangements other than thosespecifically described may be used.

The prevalent designs in contemporary small molecule and polymerphotosensitive cells utilize a transparent non-conductive material forsubstrate 110 and a transparent conductive film for anode 120, withincident light being radiated onto the exposed surface of the substrate.Such designs are limited in efficiency by large series resistance, muchof it due to the anode 120, which is generally made thin for reasons ofoptical transmissivity. In order to boost efficiency, there is a need toproduce low cost modules with lower resistance contacts.

Embodiments of the present invention include a method for forming anorganic photosensitive device having a transparent substrate-sideelectrode with improved conductivity, as well as the resulting device.

FIGS. 9A-9F illustrate a method for improving conductivity. In FIG. 9A,a mask layer 930 is formed on a transparent film comprising a firstelectrically conductive material 920 arranged on an electricallynon-conductive, transparent substrate 110. Depending upon the particularapplication, the transparent film may be continuous or patterned.Examples of a continuous film include a sheet or a coating in physicalcontact with the substrate 110. The transparent film may serve as anelectrode layer, or may serve as one layer of a compound electrode. Themask layer 930 may be made of photoresist or any other material that iseasily patterned and appropriate for use as a sacrificial layer.

The first material 920 may comprise a metal or metal substitute, asdescribed above for electrodes and Schottky contacts. Among otherthings, examples include a conductive oxide or polymer, a thin coatingof metal (as defined above to include metal alloys), or a dopedsemiconductor. An example of a conductive oxide is a degeneratesemiconductor oxide, such as indium tin oxide (ITO), gallium indium tinoxide (GITO), and zinc indium tin oxide (ZITO). In particular, ITO is ahighly doped degenerate n⁺ semiconductor with a bandgap of approximately3.2 eV, rendering it transparent to wavelengths greater thanapproximately 3900 Å. An example of suitable transparent polymer ispolyaniline (PANI) and its chemical relatives.

As described above, the transparent substrate 110 may be rigid orflexible, but rigid is generally preferred for this structure to avoidpeeling or separation of layers and short-circuits, in the finisheddevice. The substrate may also be planar or non-planar.

In FIG. 9B, the mask layer 930 is patterned (e.g., photolithography) tocreate reentrant openings 932 having sloping sides which narrowapproaching substrate 110. An example pattern for mask layer 930 isillustrated in FIGS. 10A and 10B. Although it is not required, a benefitof forming the openings 932 as a connected pattern is that wiring issimplified in the finished device.

In FIG. 9C, a second electrically conductive material 924 is depositeddirectly onto the first material 920 exposed in the reentrant openings932 of the mask 930, at least partially filling the openings 932. Thesecond material 924 may have a conductivity that is the same, or similarto the conductivity of the first material 920, but preferably is moreconductive. Examples of the second material 924 include the metal andmetal substitutes as described above. The second material 924 may be thesame as the first material 920, or may be different.

In FIG. 9D, the sacrificial mask layer 930 is stripped away, leavingbehind reentrant structures of the second material 924. Reentrant, asused herein, refers to a geometry where the sides of a structure aredirected inward, a cross-section of the structure narrowing toward itsbase (i.e., substrate 110).

In FIG. 9E, one or more organic layers 950 are deposited. These organiclayers 950 include the layer or layers which will be the photoactiveregion in the finished device. If a donor-acceptor heterojunction is tobe formed, organic layers 950 may include donor layers 152, 252,acceptor layers 154, 254, and/or a mixed layer 153, forming a planar,mixed, bulk, or hybrid heterojunction. If a Schottky junction is to beformed, the organic layers 950 may include the photoconductive material358.

In FIG. 9F, a third electrically conductive material 970 isdirectionally deposited over the organic material(s) 950 depositedbetween the reentrant structures 924. Examples of the third material 970include metal and metal substitutes as described above. By directionallydepositing between the reentrant structures of the second material 924,deposition is self aligning. The resulting gaps 975 prevent shortsbetween the transparent film and reentrant structures and the thirdmaterial 970. FIGS. 11A and 11B illustrate the corresponding results ifusing the mask from FIGS. 10A and 10B.

Even if the first material 920 and the second material 924 are the same,the reentrant structures of the second material 924 lowers resistanceand provide for increased current densities, in comparison to using thetransparent film of the first material 920 alone.

It is not necessary to cap the device. The cells may be wired up byoverlaying the electrodes during patterning and deposition. However, ifcapping or encapsulating is desired, any number of techniques may beused. For example, a coating (e.g. epoxy) may be added after theelectrodes are wired. Preferably, the processes used to deposit andpattern any capping material is selected to minimize the thermal andreactive impact on the organic layers 950.

As described above, additional layers not illustrated in the figures mayalso be deposited. For example, a reflective coating may be depositedover the third conductive material 970; a smoothing layer (122) may bedeposited over the first conductive material 920; an anti-reflectivecoating may be added to the bottom of the substrate 110 and/or orbetween the substrate 110 and the first conductor 920; and an excitonblocking layer (156) may be formed above or below a donor-acceptorheterojunction as part of the organic layers 950. The third conductivematerial 970 may serve as an electrode layer, or may serve as one layerof a compound electrode.

The self-aligning deposition steps may be used to create a tandem cell.The reentrant structures 924 may be made tall enough (much thicker thanthe total organic layer thickness in the tandem cell) to prevent shorts.By iteratively repeating process steps, additional photoactive regionsand one or more intervening charge recombination layers or chargetransfer layers may be added over the organic layer(s) 950, creating atandem cell including layers as discussed above with FIG. 7.

Similarly, a self-aligning iterative process may be used to include anintermediate electrode and an additional photoactive region, asdiscussed above with FIG. 8, utilizing the self-aligning process to formthe intermediate electrode so as to not have direct contact with thereentrant structures 924. However, as opposed to the normalserial-stacking tandem cell, forming a parallel-connected tandem cellincludes creation of an external contact to the intermediate electrode.Additional reentrant structures may optionally be created to contact theintermediate electrode, depending on the thickness and conductivity ofthe material used for this intermediate electrode.

Conventionally, the first conductive material 920 becomes the anode andthe third conductive material 970 becomes the cathode in the finisheddevice. However, an inverted device may be formed by the same process,with the anode on top and the cathode on the bottom. Although describedabove in the anode-context, smoothing layer 122 may be included in aninverted device to smooth the cathode.

As described above, organic photosensitive devices of the presentinvention may be used to generate electrical power from incidentelectromagnetic radiation (e.g., photovoltaic devices) or may be used todetect incident electromagnetic radiation (e.g., a photodetector orphotoconductor cell).

Specific examples of the invention are illustrated and/or describedherein. However, it will be appreciated that modifications andvariations of the invention are covered by the above teachings andwithin the purview of the appended claims without departing from thespirit and scope of the invention.

1. A method comprising: providing a transparent film comprising a firstelectrically conductive material, arranged on a transparent substrate;depositing and patterning a mask over the first electrically conductivematerial, such that openings in the mask have sloping sides which narrowapproaching the substrate; depositing a second electrically conductivematerial directly onto the first electrically conductive materialexposed in the openings of the mask, at least partially filling theopenings; stripping the mask, leaving behind reentrant structures of thesecond electrically conductive material which were formed by saiddepositing in the openings of the mask; after said stripping, depositinga first organic material onto the first electrically conductive materialin between the reentrant structures; and depositing a third electricallyconductive material over the first organic material deposited in betweenthe reentrant structures, edges of the reentrant structures aligningdeposition so that the third electrically conductive material does notdirectly contact the first and second electrically conductive materials.2. The method of claim 1, the second electrically conductive materialbeing more conductive than the first electrically conductive material.3. The method of claim 1, wherein the first electrically conductivematerial is an oxide or a polymer.
 4. The method of claim 3, wherein theoxide is a degenerate semiconductor oxide.
 5. The method of claim 3,wherein the second electrically conductive material is a metal.
 6. Themethod of claim 1, wherein the first electrically conductive material ofthe transparent film is a transparent coating of a metal.
 7. The methodof claim 1, wherein the second electrically conductive material is ametal.
 8. The method of claim 1, further comprising electricallyinterconnecting the third electrically conductive material depositedbetween the reentrant structures.
 9. The method of claim 1, furthercomprising depositing a second organic material over the first organicmaterial, the first and second organic materials forming a planar, bulk,or hybrid donor-acceptor heterojunction.
 10. The method of claim 1,further comprising depositing a second organic material with the firstorganic material, the first and second organic materials forming amixed, bulk or hybrid donor-acceptor heterojunction.
 11. The method ofclaim 1, wherein the first electrically conductive material is metal,the transparent film and the first organic material forming aSchottky-barrier heterojunction.
 12. The method of claim 1, wherein thethird electrically conductive material is metal, the third electricallyconductive material and the first organic material forming aSchottky-barrier heterojunction.
 13. The method of claim 1, wherein saidtransparent film is a sheet in physical contact with the transparentsubstrate.